Common cranes which were reared from eggs at the Wildfowl and Wetlands Trust have built nests in southern England.
The birds, thought to be the first to nest there in 400 years, are part of a project to reintroduce the species to the Somerset Levels and Moors.
They were reared by people dressed as adult cranes who taught the youngsters skills to survive in the wild.
Four birds have flown up the Severn Estuary and are now in the area of the Slimbridge nature reserve.
Nest building is a skill which is innate and not something the young birds were taught to do, a spokesperson for the Great Crane Project said.
Although the nesting attempt may come to nothing the building of nests is being seen as a milestone for the reintroduction project.
"This beautiful bird was so iconic in Britain, yet it was almost wiped out here by hunting and the draining of our wetlands," said Amy King, Great Crane Project aviculturist and one of the crane mums.
"This is an amazing moment seeing their first nest-building attempt in this part of the world for 400 years.
"It's even more exciting knowing that these are some of the same birds we taught, by hand, to survive by themselves.
"I feel like a very proud parent."
By 1600, the species was driven to extinction from Britain by hunting and habitat loss.
A small population has been established in the Norfolk Broads since 1979.